Lighting with DMD projection offers the opportunity to provide bright and even adaptive lighting solutions for many applications. Because the DMD array is “pixel addressable”, the projected beam of light formed with a DMD device can be adaptively shaped and masked using DMD devices to suit a variety of needs.
However, obtaining the brightness required for these applications with a DMD device that is reliable and robust has not been possible using the prior known approaches. DMD lighting efficiency is limited by the efficiency of the illumination sources and by etendue limitations of the DMD device. Further, in order to obtain the brightness needed in an automotive headlamp application for example, a DMD array in the prior known arrangements would need to be “on” 100% of the time. However, when a DMD array is operated in this manner in an environment where high operating temperatures will exist, hinge memory problems and stiction problems inevitably arise in the micro-mirror devices. A prior solution to the hinge memory problems is to operate the DMD in a lowered duty cycle, for example at a 50% duty cycle. Operating the DMD at a 50% duty cycle moves the hinge in the micro mirrors from an “on” position to an “off” position in a repeating cycle, using a clock and control signals, moving the micro-mirrors every other clock cycle avoids the hinge memory or stuck mirror problems. However, the brightness obtained at the output of the lamp system is substantially reduced when the DMD is operated in this manner, as the light is only reflected out to the projection optics 50% of the time.
FIG. 1 illustrates a conventional arrangement using a DMD device to project light for illumination. The system 10 of FIG. 1 is presented to further illustrate the problems of prior known approaches. In system 10, a single light source 20 and illumination optics 22 are used to direct light from the light source 20 onto the face of a DMD device 12. The DMD device 12 is formed by micro-electromechanical system (MEMS) technology which is based in part on semiconductor device processing. A semiconductor substrate 16 is processed using semiconductor processing steps such as photolithography and other steps including deposition, patterning, etching and metallization steps. An array of micro-mirrors 14 is formed over the substrate 16. In an example process the micro-mirrors are formed of aluminum and are mounted on a hinged mechanism. The micro-mirrors are attached on a hinge and can be tilted using electronic signals applied to electrodes that control a tilt by pivoting the micro-mirrors about an axis. In an example DMD device, thousands and even millions of the micro-mirrors are formed in an array that forms a VGA, 720p or 1080p resolution imaging device, for example. When used in a lamp application, individual micro-mirrors 14 are positioned to reflect the light from the illumination optics 22 to a projection lens 18 and a beam of light is projected out of the system 10.
The micro-mirrors 14 have three individual states, a first “on” state; second a flat or parked state, and finally an “off” state. In the “on” state, the micro-mirrors 14 in FIG. 1 are tilted in a first tilted position from the flat position, due to signals on an electrode that cause the hinge to flex, and in system 10 the micro-mirrors 14 are positioned to reflect incoming light from illumination optics 22 outwards to the projection lens 18. In the “off” state, the micro-mirrors 14 are tilted in a different tilted position to reflect the light away from the projection lens 18. By varying the tilted positions using electrical control signals, the micro-mirrors 14 can be used to direct light to the projection lens 18 or the reflected light can be reflected away from the projection lens 18. The flat state is a safe position the mirrors take when no power is applied to the device. In the flat state, the micro-mirrors 14 are not tilted as no power is applied to the control electrodes.
FIG. 2 further illustrates the operation of the micro-mirrors in a DMD array. In FIG. 2, in a projection system 30 a single illustrative micro-mirror 38 illustrates the various positions used for the micro-mirrors. In the “on” state, the micro-mirror 38 is at a first tilted position, for example at +12 degrees from the vertical or flat position. The illumination source 36 is angled at −24 degrees from the zero degree position, which is aligned with the projection lens 34. Because in reflection from a mirror, the angle of incidence (AOI) of the incoming light is equal to the angle of reflection (i) of the reflected light, for a +12 degree tilt, the −24 degree angle for the illumination source results in reflected light at the zero degree position as shown in FIG. 2. The cone of reflected light labeled “on state energy” shows the reflected light directed outwards from the micro-mirror 38 at the zero degree position. Other DMD devices may provide different tilt angles, such as +/−10 degrees, or +/−17 degrees. When the micro-mirror 38 is in the “on” state, the light from the illumination source 36 is reflected as the cone of light labeled “on state energy” at zero degrees into the projection lens 34. The projected light is then output from the system 30. The micro-mirrors can also be put in a “flat” state position, when the system is not powered, and the micro-mirrors can also be put in an “off” state. In the “off” state position, the micro-mirror 14 is at a second tilted position at an angle of −12 degrees from the flat position, and in the “off” state the light that strikes the micro-mirror is reflected away from the projection lens 34, and is not output from the system 30 but instead is output into a light dump 32. In conventional projection systems the flat position of the micro-mirror 38 is not used when power to the system is applied, but is instead used when the system is not powered. The flat position is sometimes referred to as a “parked” or “safe” position for the micro-mirror 38.
Each stage of the system 30 has some losses. The light source 36 outputs light at a certain brightness. The illumination optics in an example system has an efficiency of about 85%. The DMD device consists of thousands or millions of individual micro-mirrors such as 38. The mirrors are spaced from one another and the dark spaces between the micro-mirrors do not reflect light. The DMD device also has a transparent cover that also has some transmission losses. The DMD device has an overall efficiency of about 68%. The projection optics 34 in an example system has an efficiency of about 75%. The combined efficiency from the surface of the light source to the output of the projection lens in an example system is about 43%, the multiple of the individual efficiencies for the components coupled in the light path of the system. This is shown in table 40 in FIG. 3.
To determine the possible brightness that can be obtained with a system such as the conventional system 10 in FIG. 1, simulations were performed. Two different DMD device technologies, each available from Texas Instruments, Incorporated were evaluated. A first DMD device that has a mirror array which measures 0.3 inches diagonally and which provides a wide VGA (WVGA) resolution was evaluated. A second DMD device that has a mirror array that measures 0.47 inches diagonally with 1080p resolution was evaluated. Several light sources were evaluated. The light sources evaluated and shown in FIG. 4 are commercially available LED devices obtained from OSRAM Opto Semiconductors Company. Descriptions of the OSRAM LED devices are available at the world wide web uniform resource locator address http://www.osram-os.com/osram_os/en/products/product-catalog/led-light-emitting-diodes/osram-ostar/osram-ostar-headlamp/index.jsp. These LED devices are intended for use in headlamp applications. The Osram LEDs listed in table 45 of FIG. 4 have a range of brightness from about 800 lumens to over 1500 lumens. The brightness that could be obtained from a system such as system 10 in FIG. 1 was evaluated using these LEDs with a 100% DMD duty cycle. That is, the micro-mirrors in the DMD device were assumed to always be positioned in the “on” position to provide the maximum brightness available, which requires that the micro-mirrors be positioned in the “on” state 100% of the time. However, in this configuration, the brightness that can be obtained is not sufficient for applications such as automotive headlamps. As seen in table 45 presented in FIG. 4, the maximum brightness at the output obtained in these example configurations for systems such as the system 10 in FIG. 1 was 690 lumens.
Additional challenges in the prior known approaches occur with increasing DMD temperature. In an automotive application, for example, the headlamp is subjected to the heat caused by the operation of an automotive engine, as well as the ambient temperature, and the heat generated by an illumination source. When the DMD temperature exceeds a certain operating temperature that is specified by the manufacturer, and which temperature varies with the process used to fabricate the DMD device, if operated at a 100% duty cycle, DMD hinge memory and stiction problems are certain to occur. In an example DMD device this temperature is about 65 degrees Celsius, but as manufacturing processes continue to improve this temperature rating tends to increase. Because the lamp applications may require operating in environments where the ambient temperature is quite high, the thermal budget is difficult to manage using known prior solutions. Operating the DMD in these high temperature environments can therefore lead to hinge memory and stiction failures. In the prior known approaches, avoiding the hinge memory and stiction problems necessitates use of a 50/50 duty cycle for the DMD device when temperatures are expected over the critical operating temperature, which further limits the brightness that can be achieved.
Improvements in illumination using light projection incorporating DMD devices are therefore needed to address the deficiencies and the disadvantages of the known prior approaches. Solutions are needed that are robust, that provide reliable device operation with long device life, and that are easy to implement and use.